The present invention pertains to a method and system for charging a battery which reduces the heat dissipation in the charging circuitry. More particularly, the present invention relates to methods and apparatus for reducing heat dissipation in one or more transistors that which controls the charge current of a battery cell by altering the charging cycle to provide a reduced current after the constant current stage and before constant voltage stage.
Due to the miniaturization of electronic circuits in consumer electronics, there are large numbers of devices presently in use which rely upon batteries for their power. Such battery operated consumer electronics devices include mobile phones, laptop computers, video cameras, and like electrical devices. Since the majority of these devices use rechargeable batteries, there is a large demand for rechargeable batteries and need for improving the charging characteristics thereof.
Rechargeable batteries must periodically be connected to an external charger or supply of power to be recharged. One consequence of recharging a battery is the heat produced during the charging operation. The heat due to recharging is generally undesirable for a number of reasons. For example, the heat may damage the battery or reduce its useful life, or even cause problems in the circuitry of the device itself. In extreme cases, the heat due to recharging may be a hazard to the user, or result in unsafe conditions.
There are several types of rechargeable battery cells which are in common use. Among the most commonly used rechargeable batteries are nickel-cadmium (Ni-Cd), sealed lead acid (SLA), nickel-metal-hydride (NiMH), lithium-ion (Li-ion) and lithium-polymer (Li-polymer). Each type of rechargeable battery is characterized by properties resulting from its battery chemistry and design.
Ni-Cd batteries are known to be inexpensive, but are subject to the memory effect, that is, voltage depression. The memory effect reduces the capacity of a Ni-Cd cell if the battery is not fully discharged before re-charging it. Ni-MH batteries tend to be more expensive than Ni-Cd, but have a higher charge capacity per unit of weight than Ni-Cd batteries. Furthermore, Ni-MH batteries are not subject to the pronounced memory effect of Ni-Cd batteries.
Li-ion batteries are advantageous over the aforementioned nickel-based batteries in that Li-ion batteries have a higher energy density per unit of mass. Li-ion batteries are also not subject to the memory effect that exists in other types of nickel-based battery cells, particularly Ni-Cd cells. An advantage of Li-polymer batteries is that they may be designed to be very thin, and even exhibit some flexibility instead of being structurally rigid. However, Li-polymer batteries are fairly high cost, relative to non-lithium battery technologies.
Another battery technology worth noting is that of sealed lead-acid (SLA) batteries. SLA batteries are based on well known lead-acid battery technology. SLA batteries are relatively low cost, but tend to be relatively heavy and cumbersome as compared to other types of batteries.
FIG. 1 is a conventional battery charging system 100. The charging system 100 typically includes a power supply 110 and a battery charger assembly 120 in connection with the battery 130. The charging system 100 may be configured so as to have the negative terminal of the battery 130 and the power supply 110 both connected to a ground potential, as shown in FIG. 1. Alternatively, the battery charger assembly 120 may be provided with connections to separately connect the negative terminal of the battery 130 to a negative terminal of the power supply 110 (not shown), with no need for use of a ground return.
The battery charger assembly 120 receives current and voltage inputs from the power supply 110, and in turn, provides current and voltage to the battery 130 in accordance with a conventional charging scheme. The constant current-constant voltage (CC-CV) charging operation is typically the conventional charging scheme which is used to recharge batteries.
FIG. 2 depicts the typical current and voltage profile of a CC-CV charging operation. Rechargeable battery cells are often charged using such a CC-CV two-stage charging process in which the charger first provides a constant current, I.sub.CC, which has an associated voltage V.sub.CC. The charging process is then completed during a CV stage by providing a constant voltage, V.sub.CV, which has an associated current C.sub.CV.
During the CC stage of a conventional CC-CV charging process, the constant current I.sub.CC is applied to the battery until the cell approaches its rated voltage, V.sub.MAX, sometimes referred to as the maximum voltage. The cell voltage steadily increases during the CC stage until the fully-charged cell voltage is reached. During the CV stage of a conventional CC-CV charging process, a constant voltage equal to the fully-charged cell voltage is applied to the battery until the battery is fully charged. A battery is characterized by a rated voltage, which is often defined in the specifications provided by the manufacturer of the battery. The rated voltage, which may also be referred to as the specified charging voltage or the rated charge voltage, is the maximum recommended voltage for charging the battery. The rated voltage depends upon the battery chemistry and other design parameters of the battery. Typically, the point at which the charging process transitions from the CC stage to the CV stage occurs when the charging voltage during the CC stage reaches the rated voltage of the battery.